tōō 藤黄

Also kusashiō 草雌黄, abbreviated to *shiō 雌黄. Gamboge. A yellowish-red gum resin taken from the tree kaitōju 海藤樹 (Garcinia Glusiaceae), which grows wild in India, Thailand, and other southeast Asian countries. Gamboge turns bright yellow in water, forming an emulsion which was used as a watercolor paint *mizu-enogu 水絵具. 

The oldest examples of its use in Japan can be found on the Nara period E-ingakyō 絵因果経 (Illustrated Sutra of Cause and Effect), where gamboge is used over white lead *enpaku 鉛白 to give a yellow color.